Context. Asteroseismic determinations of structural parameters of hot B subdwarfs (sdB) have been carried out for more than a decade now. These analyses rely on stellar models whose reliability for the ... [more ▼]

Context. Asteroseismic determinations of structural parameters of hot B subdwarfs (sdB) have been carried out for more than a decade now. These analyses rely on stellar models whose reliability for the required task needs to be evaluated critically. Aims. We present new models of the so-called third generation (3G) dedicated to the asteroseismology of sdB stars, in particular to long-period pulsators observed from space. These parameterized models are complete static structures suitable for analyzing both p- and g-mode pulsators, contrary to the former second generation (2G) models that were limited to p-modes. While the reliability of the 2G models has been successfully verified in the past, this important test still has to be conducted on the 3G structures. Methods. The close eclipsing binary PG 1336−018 provides a unique opportunity to test the reliability of hot B subdwarf models. We compare the structural parameters of the sdB component in PG 1336−018 obtained from asteroseismology based on the 3G models, with those derived independently from the modeling of the reflection/irradiation effect and the eclipses observed in the light curve. Results. The stellar parameters inferred from asteroseismology using the 3G models are found to be remarkably consistent with both the preferred orbital solution obtained from the binary light curve modeling and the updated spectroscopic estimates for the surface gravity of the star. The seismology gives M∗ = 0.471 ± 0.006 M⊙ , R∗ = 0.1474 ± 0.0009 R⊙ , and log g = 5.775 ± 0.007, while orbitology leads to M∗ = 0.466 ± 0.006 M⊙ , R∗ = 0.15 ± 0.01 R⊙ , log g = 5.77 ± 0.06, and spectroscopy yields log g = 5.771 ± 0.015. In comparison seismology from a former analysis based on the 2G models gave very similar results with M∗ = 0.459 ± 0.005 M⊙ , R∗ = 0.151±0.001 R⊙, and log g = 5.739±0.002. We also show that the uncertainties on the input physics included in stellar models have no noticeable impact, at the current level of accuracy, on the structural parameters derived by asteroseismology. Conclusions. The stellar models (both of second and third generation) presently used to carry out quantitative seismic analyses of sdB stars are reliable for the task. The stellar parameters inferred by this technique, at least for those that could be tested (M∗, R, and log g), appear to be both very precise and accurate, as no significant systematic effect has been found. [less ▲]

Subdwarfs B (sdB) stars are hot (Teff=20,000-40,000 K) and compact (log g= 5.0-6.2) evolved objects that form the very hot end of the horizontal branch, the so-called Extreme Horizontal Branch (EHB). Understanding the formation of sdB stars is one of the remaining challenges of stellar evolution theory. Competing scenarios have been proposed to account for the existence of such evolved objects, and give quite different mass distributions for resulting sdB stars. Detailed asteroseismic analyses, including mass estimates, of 15 pulsating hot B subdwarfs have been published since a decade. The masses have also been reliably determined by light curve modeling and spectroscopy for 7 sdB components of eclipsing or reflection binaries. I will present in the talk the empirical mass distributions of sdB stars on the basis of these samples. I will discuss how these empirical mass distributions, although still based on small-number statistics, compare with the expectations of stellar evolution theory. In particular, the two He-white dwarfs merger scenario does not seem to be the dominant channel to form isolated sdB stars, while the post-red giant branch scenario is reinforced. This opens new questions on the extreme mass loss of red giants to form extreme horizontal branch stars, possibly in connection with the recently discovered close planets orbiting sdB stars. [less ▲]

Subdwarf B (sdB) stars are hot, compact, and evolved objects that form the very hot end of the horizontal branch, the so-called Extreme Horizontal Branch (EHB). Understanding the formation of sdB stars is ... [more ▼]

Subdwarf B (sdB) stars are hot, compact, and evolved objects that form the very hot end of the horizontal branch, the so-called Extreme Horizontal Branch (EHB). Understanding the formation of sdB stars is one of the remaining challenges of stellar evo- lution theory. Several scenarios have been proposed to account for the existence of such objects, made of He-burning core surrounded by very thin H-rich envelope. They give quite different theoretical mass distributions for the resulting sdB stars. Detailed astero- seismic analyses, including mass estimates, of 15 pulsating hot B subdwarfs have been published since a decade. The masses have also been reliably determined by light curve modeling and spectroscopy for 7 sdB components of eclipsing and/or reflection effect binaries. These empirical mass distributions, although based on small-number statistics, can be compared with the expectations of stellar evolution theory. In particular, the two He white dwarfs merger scenario does not seem to be the dominant channel to form iso- lated sdB stars, while the post-red giant branch scenario is reinforced. This opens new questions on extreme mass loss of red giants to form EHB stars, possibly in connection with the recently discovered close substellar companions and planets orbiting sdB stars. [less ▲]

Nonradial pulsations in Extreme Horizontal Branch stars (also known as hot B subdwarfs or sdB stars) offer strong opportunities to study through asteroseismology the structure and internal dynamics of stars in this intermediate stage of stellar evolution. Most sdB stars directly descend from former red giants and are expected to evolve straight into white dwarfs after core helium exhaustion. They thus represent the most direct link between these two stages. Their properties should therefore reflect both the outcome of the core evolution of red giant stars and the initial state for a fraction of the white dwarfs. We review the status of this field after a decade of efforts to exploit both p-mode and g-mode pulsating sdB stars as asteroseismic laboratories. From the discoveries of these two classes of pulsators in 1997 and 2003, respectively, up to the current epoch of data gathering of unprecedented quality from space, a lot of progress has been made in this area and prospects for future achievements look very promising. [less ▲]

We present a brief summary of what is currently known about white dwarf stars, with an emphasis on their evolutionary and internal properties. As is well known, white dwarfs represent the end products of ... [more ▼]

We present a brief summary of what is currently known about white dwarf stars, with an emphasis on their evolutionary and internal properties. As is well known, white dwarfs represent the end products of stellar evolution for the vast majority of stars and, as such, bear the signatures of past events (such as mass loss, mixing phases, loss and redistribution of angular momentum, and thermonuclear burning) that are of essential importance in the evolution of stars in general. In addition, white dwarf stars represent ideal testbeds for our understanding of matter under extreme conditions, and work on their constitutive physics (neutrino production rates, conductive and radiative opacities, interior liquid/solid equations of state, partially ionized and partially degenerate envelope equations of state, diffusion coefficients, line broadening mechanisms) is still being actively pursued. Given a set of constitutive physics, cooling white dwarfs can be used advantageously as cosmochronometers. Moreover, the field has been blessed by the existence of four distinct families of pulsating white dwarfs, each mapping a different evolutionary phase, and this allows the application of the asteroseismological method to probe and test their internal structure and evolutionary state. We set the stage for the reviews that follow on cooling white dwarfs as cosmochronometers and physics laboratories, as well as on the properties of pulsating white dwarfs and the asteroseismological results that can be inferred. [less ▲]

In light of the exciting discovery of g-mode pulsations in extremely low-mass, He-core DA white dwarfs, we report on the results of a detailed stability survey aimed at explaining the existence of these ... [more ▼]

In light of the exciting discovery of g-mode pulsations in extremely low-mass, He-core DA white dwarfs, we report on the results of a detailed stability survey aimed at explaining the existence of these new pulsators as well as their location in the spectroscopic Hertzsprung–Russell diagram. To this aim, we calculated some 28 evolutionary sequences of DA models with various masses and chemical layering. These models are characterized by the so-called ML2/α = 1.0 convective efficiency and take into account the important feedback effect of convection on the atmospheric structure. We pulsated the models with the nonadiabatic code MAD, which incorporates a detailed treatment of time-dependent convection. On the other hand, given the failure of all nonadiabatic codes, including MAD, to account properly for the red edge of the strip, we resurrect the idea that the red edge is due to energy leakage through the atmosphere. We thus estimated the location of that edge by requiring that the thermal timescale in the driving region—located at the base of the H convection zone—be equal to the critical period beyond which l = 1 g-modes cease to exist. Using this approach, we find that our theoretical ZZ Ceti instability strip accounts remarkably well for the boundaries of the empirical strip, including the low-gravity, low-temperature regime where the three new pulsators are found. We also account for the relatively long periods observed in these stars, and thus conclude that they are true ZZ Ceti stars, but with low masses. [less ▲]

Aims. The determination of the location of the theoretical ZZ Ceti instability strip in the log g − Teff diagram has remained a challenge over the years due to the lack of a suitable treatment for ... [more ▼]

Aims. The determination of the location of the theoretical ZZ Ceti instability strip in the log g − Teff diagram has remained a challenge over the years due to the lack of a suitable treatment for convection in these stars. For the first time, a full nonadiabatic approach including time-dependent convection is applied to ZZ Ceti pulsators, and we provide the appropriate details related to the inner work- ings of the driving mechanism at work. Methods. We used the nonadiabatic pulsation code MAD with a representative evolutionary sequence of a 0.6 M⊙ DA white dwarf. This sequence is made of state-of-the-art models that include a detailed modeling of the feedback of convection on the atmospheric structure. The assumed convective efficiency in these models is the so-called ML2/α = 1.0 version. We also carried out, for comparison purposes, nonadiabatic computations within the frozen convection approximation, as well as calculations based on models with standard grey atmospheres. Results. We find that pulsational driving in ZZ Ceti stars is concentrated at the base of the superficial H convection zone, but at depths, near the blue edge of the instability strip, somewhat larger than those obtained with the frozen convection approach. Despite the fact that this approach is formally invalid in such stars, particularly near the blue edge of the instability strip, the predicted boundaries are not dramatically different in both cases. The revised blue edge for a 0.6 M⊙ model is found to be around Teff = 11,970 K, some 240 K hotter than the value predicted within the frozen convection approximation, in rather good agreement with the empirical value. On the other hand, our predicted red edge temperature for the same stellar mass is only about 5600 K (80 K hotter than with the frozen convection approach), much lower than the observed value. Conclusions. We correctly understand the development of pulsational instabilities of a white dwarf as it cools at the blue edge of the ZZ Ceti instability strip. Our current implementation of time-dependent convection however still lacks important ingredients to fully account for the observed red edge of the strip. We will explore a number of possibilities in the future papers of this series. [less ▲]

We present the results of about a decade of efforts toward building an empirical mass distribution for hot B subdwarf stars on the basis of asteroseismology. So far, our group has published detailed ... [more ▼]

We present the results of about a decade of efforts toward building an empirical mass distribution for hot B subdwarf stars on the basis of asteroseismology. So far, our group has published detailed analyses pertaining to 16 pulsating B subdwarfs, including estimates of the masses of these pulsators. Given that measurements of the masses of B subdwarfs through more classical methods (such as full orbital solutions in binary stars) have remained far and few, asteroseismology has proven a tool of choice in this endeavor. On the basis of a first sample of 15 pulsators, we find a relatively sharp mass distribution with a mean mass of 0.470 M⊙, a median value of 0.470 M⊙, and a narrow range 0.441−0.499 M⊙ containing some 68.3% of the stars. We augmented our sample with the addition of seven stars (components of eclipsing binaries) with masses reliably established through light curve modeling and spectroscopy. The new distribution is very similar to the former one with a mean mass of 0.470 M⊙, a median value of 0.471 M⊙, and a slightly wider range 0.439−0.501 M⊙ containing some 68.3% of the stars. Although still based on small-number statistics, our derived empirical mass distribution compares qualitatively very well with the expectations of stellar evolution theory. [less ▲]

Planets that orbit their parent star at less than about one astronomical unit (1AU is the Earth-Sun distance) are expected to be engulfed when the star becomes a red giant. Previous observations have ... [more ▼]

Planets that orbit their parent star at less than about one astronomical unit (1AU is the Earth-Sun distance) are expected to be engulfed when the star becomes a red giant. Previous observations have revealed the existence of post-red-giant host stars with giant planets orbiting as close as 0.116AU or with brown dwarf companions in tight orbits, showing that these bodies can survive engulfment. What has remained unclear is whether planets can be dragged deeper into the red-giant envelope without being disrupted and whether the evolution of the parent star itself could be affected. Here we report the presence of two nearly Earth-sized bodies orbiting the post-red-giant, hot B subdwarf star KIC 05807616 at distances of 0.0060 and 0.0076AU, with orbital periods of 5.7625 and 8.2293 hours, respectively. These bodies probably survived deep immersion in the former red-giant envelope. They may be the dense cores of evaporated giant planets that were transported closer to the star during the engulfment and triggered the mass loss necessary for the formation of the hot B subdwarf, which might also explain how some stars of this type did not form in binary systems. [less ▲]

Detailed asteroseismic analyses of 15 pulsating B subdwarfs have been published since a decade, including estimates of the masses of these stars. We present in this talk the empirical mass distribution ... [more ▼]

Detailed asteroseismic analyses of 15 pulsating B subdwarfs have been published since a decade, including estimates of the masses of these stars. We present in this talk the empirical mass distribution for hot B subdwarfs on the basis of this sample. We find a sharp mass distribution with a mean mass of 0.470 Msun, a median value of 0.471 Msun, and 68.3% of the stars fall in the narrow range of mass 0.441-0.499 Msun. In a second experiment, we augment our sample with the addition of 5 hot B subdwarfs components of eclipsing binaries, with masses reliably determined by light curve modeling and spectroscopy. The new mass distribution is very similar to the former one with a mean mass of 0.469 Msun, a median value of 0.471 Msun, and a range 0.436-0.501 Msun containing 68.3% of the stars. We also discuss in this talk how these empirical mass distributions, although still based on small-number statistics, compare with the expectations of stellar evolution theory. [less ▲]

We present a seismic analysis of the pulsating subdwarf B star KPD 0629-0016 on the basis of the long-period, gravity-mode pulsations uncovered by CoRoT. Thanks to space- based facilities, g-mode ... [more ▼]

We present a seismic analysis of the pulsating subdwarf B star KPD 0629-0016 on the basis of the long-period, gravity-mode pulsations uncovered by CoRoT. Thanks to space- based facilities, g-mode seismology can now be exploited quantitatively for stars on the ex- treme horizontal branch, an objective undermined so far by the limitations of ground-based observations. The optimal seismic model offers an excellent fit, with a relative dispersion of 0.23%, to the 18 observed periods identified with theoretical modes of degrees l =1 and 2. The inferred structural parameters for KPD 0629–0016 include the total stellar mass, the thickness of the H-rich envelope, and, thanks to the sensitivity of g-modes, the size and the composition of the convective core. Our results suggest that extra mixing processes shape the helium-burning cores, that are representative of all horizontal branch stars in general, an intermediate and underrated stage of stellar evolution. [less ▲]

Context. Contemporary high precision photometry from space provided by the Kepler and CoRoT satellites generates significant breakthroughs in terms of exploiting the long-period, g-mode pulsating hot B subdwarf (sdBVs) stars with asteroseismology. Aims: We present a detailed asteroseismic study of the sdBVs star KIC02697388 monitored with Kepler, using the rich pulsation spectrum uncovered during the ~27-day-long exploratory run Q2.3. Methods: We analyse new high-S/N spectroscopy of KIC02697388 using appropriate NLTE model atmospheres to provide accurate atmospheric parameters for this star. We also reanalyse the Kepler light curve using standard prewhitening techniques. On this basis, we apply a forward modelling technique using our latest generation of sdB models. The simultaneous match of the independent periods observed in KIC02697388 with those of models leads objectively to the identification of the pulsation modes and, more importantly, to the determination of some of the parameters of the star. Results: The light curve analysis reveals 43 independent frequencies that can be associated with oscillation modes. All the modulations observed in this star correspond to g-mode pulsations except one high-frequency signal, which is typical of a p-mode oscillation. Although the presence of this p-mode is surprising considering the atmospheric parameters that we derive for this cool sdB star (Teff = 25 395 ± 227 K, log g = 5.500 ± 0.031 (cgs), and log N(He) /N(H) = -2.767 ± 0.122), we show that this mode can be accounted for particularly well by our optimal seismic models, both in terms of frequency match and nonadiabatic properties. The seismic analysis leads us to identify two model solutions that can both account for the observed pulsation properties of KIC02697388. Despite this remaining ambiguity, several key parameters of the star can be derived with stringent constraints, such as its mass, its H-rich envelope mass, its radius, and its luminosity. We derive the properties of the core proposing that it is a relatively young sdB star that has burnt less than ~34% (in mass) of its central helium and has a relatively large mixed He/C/O core. This latter measurement is in line with the trend already uncovered for two other g-mode sdB pulsators analysed with asteroseismology and suggests that extra mixing is occurring quite early in the evolution of He cores on the horizontal branch. Conclusions: Additional monitoring with Kepler of this particularly interesting sdB star should reveal the inner properties of KIC02697388 and provide important information about the mode driving mechanism and the helium core properties. [less ▲]

Asteroseismology is a recent branch of astrophysics that studies the interiors of stars by the interpretation of their pulsation spectra. A wide variety of stars exhibit pulsations, including gravity ... [more ▼]

Asteroseismology is a recent branch of astrophysics that studies the interiors of stars by the interpretation of their pulsation spectra. A wide variety of stars exhibit pulsations, including gravity-modes (driven by buoyancy) that usually penetrate deep inside the stars. By probing these deep layers unreachable from classical observations, the g-mode oscillations bring invaluable information for stellar evolution and astrophysics in general. I will illustrate in my talk the power of g-mode asteroseismology by the example of Extreme Horizontal Branch stars, that are on an intermediate stage of evolution, and show how g-modes allow us to determine the properties of the cores in these stars, including their convective characteristics, size and composition. [less ▲]

Context. The asteroseismic exploitation of long period, g-mode hot B subdwarf pulsators (sdBVs), undermined so far by limitations associated with ground-based observations, has now become possible, thanks ... [more ▼]

Context. The asteroseismic exploitation of long period, g-mode hot B subdwarf pulsators (sdBVs), undermined so far by limitations associated with ground-based observations, has now become possible, thanks to high quality data obtained from space such as those recently gathered with the CoRoT (COnvection, ROtation, and planetary Transits) satellite. Aims. We propose a detailed seismic analysis of the sdBVs star KPD 0629-0016, the first compact pulsator monitored with CoRoT, using the g-mode pulsations recently uncovered by that space-borne observatory during short run SRa03. Methods. We use a forward modeling approach on the basis of our latest sdB models, which are now suitable for the accurate com- putation of the g-mode pulsation properties. The simultaneous match of the independent periods observed in KPD 0629-0016 with those of the models leads objectively to the identification of the pulsation modes and, more importantly, to the determination of the structural and core parameters of the star. Results. The optimal model we found closely reproduces the 18 observed periods retained in our analysis at a 0.23% level on av- erage. These are identified as low-degree (l = 1 and 2), intermediate-order (k = −9 through −74) g-modes. The structural and core parameters for KPD 0629-0016 are the following (formal fitting errors only): Teff = 26 290 ± 530 K, log g = 5.450 ± 0.034, M∗ = 0.471 ± 0.002 M⊙, log (Menv/M∗) = −2.42 ± 0.07, log (1 − Mcore/M∗) = −0.27 ± 0.01, and Xcore(C+O) = 0.41 ± 0.01. We addition- ally derive an age of 42.6 ± 1.0 Myr after the zero-age extreme horizontal branch, the radius R = 0.214 ± 0.009 R⊙, the luminosity L = 19.7 ± 3.2 L⊙, the absolute magnitude MV = 4.23 ± 0.13, the reddening index E(B − V) = 0.128 ± 0.023, and the distance d = 1190 ± 115 pc. Conclusions. The advent of high-precision time-series photometry from space with instruments like CoRoT now allows as demon- strated with KPD 0629-0016 the full exploitation of g-modes as deep probes of the internal structure of these stars, in particular for determining the mass of the convective core and its chemical composition. [less ▲]

In 2007, a companion with planetary mass was found around the pulsating subdwarf B star V391 Pegasi with the timing method, indicating that a previously undis- covered population of substellar companions ... [more ▼]

In 2007, a companion with planetary mass was found around the pulsating subdwarf B star V391 Pegasi with the timing method, indicating that a previously undis- covered population of substellar companions to apparently single subdwarf B stars might exist. Following this serendip- itous discovery, the EXOTIME (http://www.na.astro.it/ ~silvotti/exotime/) monitoring program has been set up to follow the pulsations of a number of selected rapidly pul- sating subdwarf B stars on time scales of several years with two immediate observational goals: (1) determine P ̇ of the pulsational periods P (2) search for signatures of substellar companions in O– C residuals due to periodic light travel time variations, which would be tracking the central star’s companion- induced wobble around the centre of mass These sets of data should therefore, at the same time, on the one hand be useful to provide extra constraints for classical asteroseismological exercises from the P ̇ (comparison with “local” evolutionary models), and on the other hand allow one to investigate the preceding evolution of a target in terms of possible “binary” evolution by extending the otherwise unsuccessful search for companions to potentially very low masses. While timing pulsations may be an observationally ex- pensive method to search for companions, it samples a dif- ferent range of orbital parameters, inaccessible through or- bital photometric effects or the radial velocity method: the latter favours massive close-in companions, whereas the timing method becomes increasingly more sensitive toward wider separations. In this paper we report on the status of the on-going ob- servations and coherence analysis for two of the currently five targets, revealing very well-behaved pulsational charac- teristics in HS 0444+0458, while showing HS 0702+6043 to be more complex than previously thought. [less ▲]

The hot pulsating sdB star PG 1605+072 exhibits uncommon spectroscopic and pulsation properties, and is one of the biggest challenge in the field of sdB star modeling. Two hypotheses have been proposed to ... [more ▼]

The hot pulsating sdB star PG 1605+072 exhibits uncommon spectroscopic and pulsation properties, and is one of the biggest challenge in the field of sdB star modeling. Two hypotheses have been proposed to explain its unusually rich pulsation spectrum. The first is the natural explanation of a fast-rotating pulsator, which lifts the (2 l+1)-fold degeneracy of the frequency components. Another approach, where PG 1605+072 can be seen as a slow rotator, considers that the numerous low amplitude frequency components are second- and third-order harmonics and nonlinear combinations of the highest amplitude frequencies. We investigated the two hypotheses in the light of asteroseismology, using our latest tools—including pulsation codes that incorporate star rotation and new generation complete sdB models. The results of both approaches are presented, showing interesting results and raising new questions for our understanding of this mysterious sdB star. [less ▲]

We present a seismic analysis of the pulsating hot B subdwarf KPD 1943+4058 (KIC 005807616) on the basis of the long-period, gravity-mode pulsations recently uncovered by Kepler. This is the first time ... [more ▼]

We present a seismic analysis of the pulsating hot B subdwarf KPD 1943+4058 (KIC 005807616) on the basis of the long-period, gravity-mode pulsations recently uncovered by Kepler. This is the first time ... [more ▼]

We present a seismic analysis of the pulsating hot B subdwarf KPD 1943+4058 (KIC 005807616) on the basis of the long-period, gravity-mode pulsations recently uncovered by Kepler. This is the first time that g-mode seismology can be exploited quantitatively for stars on the extreme horizontal branch, all previous successful seismic analyses having been confined so far to short-period, p-mode pulsators. We demonstrate that current models of hot B subdwarfs can explain quite well the observed g-mode periods, while being consistent with independent constraints provided by spectroscopy. We identify the 18 pulsations retained in our analysis as low-degree (l = 1 and 2), intermediate-order (k = −9 through −58) g-modes. The periods (frequencies) are recovered, on the average, at the 0.22% level, which is comparable to the best results obtained for p-mode pulsators. We infer the following structural and core parameters for KPD 1943+4058 : Teff = 28,050 ± 470 K, log g = 5.520 ± 0.029, M∗ = 0.4964 ± 0.0013 M⊙, log (Menv/M∗) = −2.552 ± 0.070, log (1−Mcore/M∗) = −0.366 ± 0.010, and Xcore(C+O) = 0.2612 ± 0.0080. We additionally derive the age of the star since the Zero-Age EHB 18.4 ± 1.0 Myr, the radius R = 0.2026 ± 0.0070 R⊙, the luminosity L = 22.92 ± 3.13 L⊙, the absolute magnitude MV = 4.21 ± 0.11, the reddening index E(B − V ) = 0.094 ± 0.017, and the distance d = 1183 ± 93 pc. [less ▲]